LOW-ENERGY PHONON EXCITATIONS IN SUPERCONDUCTING RNIB2C (R = LU, Y)

Inelastic neutron-scattering techniques have been used to study the lo w-energy phonon excitations in superconducting RNi2B2C (R = Lu, Y) to further characterize the anomalous features observed by Kawano et al. [Phys. Rev. Lett. 77, 4628 (1996)] for R = Y and Stassis et al. [Phys. Rev. B 55, R8678 (1997)] for R = Lu, when these systems enter the sup erconducting ground state. We find that above T, the frequencies of th e Delta(4)[xi 00] lowest-lying acoustic and optic phonon modes decreas e with decreasing temperature, for xi close to the nesting vector xi(m ). In addition there is a shift of intensity from the upper to the low er mode, an effect characteristic of mode coupling. The observed inten sity transfer between these modes above T-c can be described satisfact orily in both compounds by a coupled-mode model. Below T-c the observe d spectrum changes dramatically: it consists of a sharp peak at approx imately 4 meV with a broad weak shoulder at higher energies. The exper imental results unambiguously show that this dramatic change is due to the onset of superconductivity in these compounds. In this temperatur e region, the results are in qualitative agreement with recent theoret ical calculations.